1. Field of the Invention
The present invention generally relates to a two-sided printing apparatus, and more particularly to an improvement in a sheet transport path for a two-sided print.
2. Description of the Prior Art
In a duplex printing apparatus, images are formed on one side of a recording sheet, such as paper, in an image transfer unit and are fixed in a fixing unit. Then the recording sheet is turned over, and refed to the image transfer unit. Thereafter, images are formed on the other side of the recording sheet, and are fixed in the fixing unit.
FIG. 1 shows a conventional two-sided printing apparatus, which comprises a photosensitive drum 1, around which are located an exposure position 2, a developing unit 3, an image transfer unit 4, a cleaner unit 5, and a uniform charger 6. When the entire width of the photosensitive drum 1 is charged by means of the uniform charger 6, and reaches the exposure position 2, an information light 8 deflected by an optical-system unit 7 is projected onto the exposure position 2, and hence an electrostatic latent image is formed on the photosensitive drum 1. Toner particles are adhered to the circumferential surface of the photosensitive drum 1 by means of the developing unit 3, so that a toner image is formed. The toner image is transferred to a sheet such as paper at the position of the image transfer unit 4. The cleaner unit 5 removes remaining toner particles and remaining charges from the photosensitive drum 1. The photosensitive drum 1 returns to the position of the uniform charger 6.
A sheet is fed from a sheet cassette 9, and transported to a pair of registration rollers 11 via a sheet path 10. Then, the sheet is positioned with respect to the toner image formed on the photosensitive drum 1, and is then transported to the image transfer unit 4. The image transfer unit 4 shown in FIG. 1 comprises a transfer charger. By means of a corona discharge, a charge having a polarity opposite to that of the toner image is applied to the sheet. Hence, the toner image on the photosensitive drum 1 adheres to the sheet.
The sheet to which the toner image has adhered is detached from the photosensitive drum 1 by means of a sheet detachment charger 12, and is then transported the fixing unit 13, in which the sheet is heated and the image is fixed on the sheet. That is a roller 13a of the fixing unit 13 on the side of the photosensitive drum 1 is a thermal roller in which a heater is installed. While the sheet to which the toner image adheres is passing between the thermal roller 13a and a pressure roller 13b, the sheet and the toner image are heated. Thereby, the toner image is melted and hardened, so that it is fixed on the sheet.
The sheet having the fixed toner image passes between an impeller 14 and an opposing roller 15 for sheet ejection when one-side print is to be performed. Then, the sheet is ejected to an ejection tray 19 by means of pairs 16, 17 and 18 of forward/reverse rollers located in an ejection transport path 24. In a two-sided print, the pairs 16, 17 and 18 of rollers are reversed when the rear end of the sheet has reached a position between the impeller 14 and the pair 16 of rollers. Thereby, the rear end of the sheet is guided between the impeller 14 and the pair 20 of rollers, and the sheet is transported to the registration rollers 11 by means of pairs 22 and 23 provided in a transport path 21 for back-surface printing.
During the above, the sheet is turned over, and transported between the fixing unit 4 and the photosensitive drum 1. When the sheet's back above-mentioned manner, a toner image on the photosensitive drum 1 is transferred to the back surface of the sheet. Then, the sheet is transported to the fixing unit 13 in which the image fixed. Thereafter, the sheet passes between the impeller 14 and the opposite roller 15, and is ejected to the ejection tray by means of the pairs 16, and 18 of rollers.
FIG. 2 is a diagram showing how a plurality of sheets are transported in the conventional two-sided printing apparatus shown in FIG. 1. S1 indicates a first sheet having a back surface on which an image is being printed. A second sheet S2 having a front surface on which an image has been printed is transported ahead of the first sheet S1. In this case, the first sheet S1 cannot be ejected because of the presence of the second sheet S2. More particularly, after the toner image is transferred to and fixed on the first sheet S1, the first sheet S1 is transported towards the ejection tray. The second sheet S2 located ahead of the first sheet S1 is transported towards the ejection tray until the rear end of the second sheet S2 passes over the impeller 14, and is moved back to the transport path 21 due to the reverse rotations of the rollers 16, 17 and 18.
Hence, the first sheet S1 having images printed on both sides thereof cannot be transported through the transport path 24 until second sheet S2 is completely moved back to the transport path 21. Hence, in practice, a sufficient distance is provided between the second sheet S2 and the first sheet S1 so that, after the two-sided print, the leading end of the first sheet S1 is located between the impeller 14 and the opposing roller 15 after the second sheet S2 is completely shunted to the transport path 21, as shown by a chained line S2'. Hence, when the second sheet S2 after the one-side print is located at the position indicated by a solid line, the first sheet S1 is located in the transport path and is far away from the second sheet S2. In this case, the first sheet S1 and the second sheet S2 are spaced apart from each other by a distance equal to at least the sheet length.
As described above, it is necessary to sufficiently separate the first and second sheets S1 and S2 from each other. This decreases the printing speed, and causes delay in ejection of the first sheet S1. The first sheet S1 after the two-sided print cannot be ejected rapidly, which increases the number of sheets in the apparatus and increases the possibility of jamming.
In order to eliminate the above disadvantages and eject the sheet after the two-sided print as rapidly as possible, the following method has been proposed. According to the proposed method, the second sheet S2 is transported to the transport path 21 at a speed higher than the speed at which the sheet is ejected. For this purpose, the rollers 16, 17 and 18 are reversed at a circumferential speed higher than the circumferential speed of forward rotation of the rollers 16, 17 and 18. With the above proposed method, it becomes possible to reduce the distance between the first sheet S1 and the second S2 traveling ahead thereof and to rapidly eject the first sheet S1 after the two-sided print. Further, it becomes possible to reduce the number of sheets in the apparatus and to reduce the possibility of jamming.
Another method has been proposed in which the opposing rollers 15 and 20 cooperating with the impeller 14 are omitted, and the impeller 14 is located between the fixing unit 13 and the rollers 16 positioned at the end of the ejection transport path 24 in such a manner that the impeller 14 does not have any opposing roller. With this arrangement, it is possible to obtain the difference between the ejection transport speed of the first sheet S1 and the shunting speed of the second sheet S2. The circumferential speed of the outer portion of the impeller 14 is set to be higher than the shunting speed of the second sheet S2. Thereby, the shunting transport speed of the second sheet S2 is higher than the ejection transport speed of the first sheet S1 without preventing the high-speed shunting operation of the second sheet S2.
Further, as shown by the chained line in FIG. 2, the transport roller pair 25 is located just below the impeller 14 in the transport path 21 for back-surface printing. As shown in FIG. 1, the portion of the transport path 21 between the impeller 14 and the transport rollers 22 is a downward slope in order to facilitate smooth and high-speed shunting movement. The downward slope is formed by an inclined frame 29 fastened to a base 27 by means of a supporting shaft 28.
When the two-sided printing apparatus is working, the frame 29 is maintained in the inclined state, in which the impeller 14 and the transport rollers 25 are positioned at the upper portion of the frame 29, and the transport roller 22 is located at the lower end thereof. The second sheet S2 moving from the ejection transport path 24 descends due to the transporting force generated by the rollers 25 as well as the weight of the second sheet S2. Hence, the second sheet can be more smoothly shunted to the transport path 21.
When a sheet jam takes place in the transport path 21, the frame 29 is pivoted about the axis of the shaft 28 in the direction indicated by the arrow and reaches a level position. In this manner, the inner portion of the apparatus can be easily accessed, and a jammed sheet can be easily removed.
However, if a sheet jam takes place in the back-surface transport path 21 where the second sheet S2 is transported after the front-surface printing is completed, the rollers 25, 22 and 23 are manually turned to the state where the frame is horizontally maintained. If the second sheet S2 slips or is torn during the removal operation, it will become difficult or impossible to remove the second sheet S2 from the transport path 21. If the removal of the jammed sheet is forced, the apparatus may be damaged.